Re: [PATCH 1/2] lkdtm: use atomic_t to replace count_lock

From: Cong Wang
Date: Thu Feb 02 2012 - 08:34:06 EST

On 02/01/2012 11:27 PM, Arnd Bergmann wrote:
On Wednesday 01 February 2012, Cong Wang wrote:
static void lkdtm_handler(void)
- unsigned long flags;
- spin_lock_irqsave(&count_lock, flags);
- count--;
printk(KERN_INFO "lkdtm: Crash point %s of type %s hit, trigger in %d rounds\n",
- cp_name_to_str(cpoint), cp_type_to_str(cptype), count);
+ cp_name_to_str(cpoint), cp_type_to_str(cptype), atomic_dec_return(&count));

- if (count == 0) {
+ if (!atomic_cmpxchg(&count, 0, cpoint_count))
- count = cpoint_count;
- }
- spin_unlock_irqrestore(&count_lock, flags);

This use is not atomic, you could have two threads doing atomic_dec_return
at the same time, and after that the value will be -1 so the atomic_cmpxchg
does not trigger.

Yeah, simply combining two atomic operations is not atomic. :-/

In order to have an atomic here, you have to use a loop around
atomic_cmpxchg, like

int old, new;
old = atomic_read(&count);
do {
new = old ? old - 1 : cpoint_count;
old = cmpxchg(&count, old, new);
} while (old != new);

I suppose you could also just keep the spinlock and move lkdtm_do_action()
outside of it?

If we still need spinlock, I think we don't need to bother atomic_t at all.
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